JPS5876810A - Light beam scanner - Google Patents

Abstract

PURPOSE:To obtain a spot of uniform size on a surface to be scanned, by setting a reflecting surface and the surface to be scanned which are conjugate geometrically and optical at right angles to a scanning direction, and thus preventing unevenness of scanning pitch and forming a beam waist. CONSTITUTION:A light beam which is wide and flat in a scanning direction and further nearly parallel is incident from a beam shaping means which is not shown in the figure to a rotary polygon mirror 11 to reach a surface SP to be scanned through a condenser lens (f.theta lens) 12 and a convex cylindrical lens 13. The reflecting surface of the rotary polygon mirror 11 and the surface SP to be scanned are conjugate geometrically and optically; a light beam traveling to the center of scanning width through the condenser lens 12 has the 1st beam waist W1 before the cylindrical lens 13 in the direction of an arrow (y) perpendicular to the scanning direction, and a light beam diffused again has the 2nd beam waist W2 of desired size on the surface to be scanned through the cylindrical lens 13, thus obtaining a uniform diameter over the overall scanning width.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical beam scanning device that scans a surface to be scanned by reflecting a light beam from a light source on a rotating polygon mirror and applying it to a surface to be scanned through a condensing lens.

As shown schematically in Figure 1, this type of light beam scanning device originally uses a light beam from a laser beam, etc., which is incident on a rotating polygon */, to pass through the mirror surface of a rotating polygon mirror l that rotates in the direction of the arrow. The light is reflected, passes through a condensing lens (f/θ lens) 3t, and connects a light spot moving at a uniform speed in the scanning direction (X direction) onto the scanned surface sp. However, if each mirror surface is not parallel to the rotation axis of the single-rotation polygon mirror l, and there are irregularities in their angles (inclination angle error),
If the light spot shifts non-uniformly in the direction perpendicular to the scanning direction (y direction) and, for example, the tilt angle error is Δθ as shown in Fig. 2, then Δd=kof・Δθ (where , f is the focal length of the condenser lens 3). Since this inclination angle error and rotational axis deviation cause pitch unevenness t of the scanning line, it is necessary to take some countermeasures. One way to do this is to improve machining accuracy and minimize the angle of inclination error, but in reality this is close to the limit in terms of machining accuracy, and even if machining could be done, it would take a lot of man-hours, making mass production difficult. The problem is that it is extremely expensive. or,
Conventionally, as shown in FIGS. 3 and 3, a light beam is incident on a rotating polygon mirror l as a linear beam parallel to the scanning direction using a cylindrical lens, etc. A cylindrical lens (or toroidal lens) placed between l1iSP! and a condenser lens 3 that corrects the linear beam and scanning position as optical conjugates in a direction perpendicular to the scanning direction (%Kaishoat-≠31!
No.). However, when using a cylindrical lens, it is not possible to obtain a uniform spot size over the entire scanning width, and a cylindrical lens! If a toroidal lens is used instead, a uniform spot size can be obtained, but new problems arise, such as the expensive rounding of the toroidal lens and the high cost of the light beam scanning device.

Furthermore, as another conventional example, as shown in FIGS. 3 and 6, a flat (FIG. 7) and nearly parallel light beam is made incident on a rotating polygon mirror l using cylindrical lenses 6 and 7, and is focused. There is also one in which a cylindrical lens If is simply provided at the threshold between the optical lens 3 and the scanned gsp. This device also has the problem of not being able to obtain a uniform spot size over the entire scanning width.

The present invention has been made in view of the above-mentioned problems, and is a light beam scanning device that reflects a light beam from a light source on a rotating polygon mirror and applies it to a surface to be scanned via a condenser lens for scanning.
A beam shaping means is provided between the light source and the rotating polygon mirror to form a light beam that is wide in the scanning direction and is not oblate and is substantially parallel, and a beam shaping means is provided near the scanned surface in a direction perpendicular to the scanning direction. An optical element having a light convergence effect is provided only in the direction of the scanning direction, and the reflective surface of the rotating polygonal mirror and the surface to be scanned are configured to have a geometrically conjugate relationship, With respect to the direction, a beam waist of i@l is generated between the condenser lens and the optical element, and a second beam waist is generated near the scanned surface, so that the beam waist is uniformly generated over the entire scanning width. This optical beam scanning device has a simple configuration and is inexpensive, which can obtain a spot size of 4 m and which does not cause pitch irregularities.

The present invention will be explained in detail below.

Figure 1 is a schematic diagram of the optical system and the light beam between the rotating polygon mirror and the surface to be scanned in the light beam scanning device according to the present invention, where (b) is a top view and (-) is the center of scanning. III
I-plane view, (-) indicates @-plane view at the scanning end. In the figure, l/ is a rotating polygon mirror, and its reflection rfJ<mirror surface is 1
A light beam that is wide in the scanning direction, is not oblate, and is nearly parallel is incident from a beam shaping means (not shown). 12 is a condensing lens (an f/θ lens is used here) that receives the reflected beam from the rotating polygon mirror //, and 13 is a convex cylinder placed near the scanned surface (usually a flat surface) sp. It's a lens. This convex cylindrical lens 13 is one of the optical elements that has a convergence effect only in one direction, and is in the scanning direction (X direction).
This lens has refractive power only in the direction perpendicular to the direction (y direction).

In the device of the present invention, the reflecting surface of the rotating polygon mirror // and the scanned surface s
p is a conjugate trap in terms of geometric optics, and the light beam that passes through the condenser lens l and heads toward the center of the scanning width has a first beam waist WI in one direction before the cylindrical lens 13 ( The light beam uniformly converges toward the scanned surface BP from the condensing lens /J in the X direction), and then diffuses again.
The cylindrical lens 13 is configured to produce a first beam waist having a desired size on the scanned surface SP. Here, as the light beam received in the pfII direction by the rotating polygon mirror /I moves from the scanning center to the scanning end,
The first beam waist in the y direction draws an arc and the cylindrical lens 1
The east diameter of the one-directional light at the point of incidence on the light beam 3 is larger than that at the center of the scan. The effective focal length of the cylindrical lens 13 is determined by the fact that the light beam is incident obliquely, so as the light beam moves away from the scanning center, the second y of the light beam after passing through the cylindrical lens 13 increases. The direction beam waist is the scanned surface sp
, and the beam waist diameter at that position becomes smaller than that at the center of the scan. After passing through the second y-direction beam waist, the multiplication beam diverges in the y-direction and enters the scanned surface sp with a slightly increased luminous flux diameter.

Here, rotating polygon mirror l/, condensing lens Fuco 1, cylindrical lens 1
3. By appropriately determining the relative position of the scanned surface sp, the focal length, the diameter of the light beam incident on this system, the radius of wavefront curvature, etc., the diameter of the spot in one direction can be adjusted to the entire scanning width on the scanned surface BP. The number of chickens can be made to be almost uniform.

The following is a relational expression that comes into consideration when understanding the convergence and divergence of a light beam and when tracking the entire light beam to obtain a desired spot. However, it is assumed that a laser is used as the light source and that the light intensity distribution within the light beam is Gaussian.

(1), (2) Equation H, in the beam waist, the radius (defined as the position where the central intensity is l/-) is ω in the wave and λ. A light beam has a radius of light flux ω and a radius of wavefront curvature R1 at a position 2 apart from the beam waist. Conversely, at a certain position, the radius ω of the light beam and the radius of wavefront curvature R
If is given, from that position, the distance to the beam waist 2· and the beam radius ωo at the beam waist can be determined based on the following equation.

"=/+(,,)" ...(3) From equation (3), when R is large and ω is small, the beam waist is closer to the focal point ze = R determined by geometric optics. It can be seen that this occurs in Further, when a multiplication beam enters an optical element having refractive power such as a lens, only the wavefront radius of curvature undergoes transformation as shown by the following equation at the refractive surface.

l n < −−−)=n′(L−′−z)・・・・−・(
5) r8 r8 Here, "wtl' is the refractive index of the medium before and after the refracting surface, r is the radius of curvature of the refracting surface, and S%S' is the radius of curvature of the wavefront before and after the refracting surface. Furthermore, in equation (5), Alternatively, it is also possible to roughly calculate using the following equation using the focal length f of the lens.

In this way, conditions such as convergence and divergence of the light beam emitted from the laser can be known from equations (1) to (6).

Spot uniformity (spot E) must be perfectly circular
7! The conditions for obtaining a cylindrical lens 13 and the scanning f-plane sp according to the desired spot diameter are as follows:
As the distance of the spot becomes smaller and the spot diameter becomes smaller,
The cylindrical lens 13 is required to be close to the scanned surface sp.

Next, specific examples will be shown.

Example / Light source: H・-N・Laser condenser lens: t-3to, (t・0 lens) Cylindrical lens: t=10° Cylindrical lens: Pressure between surfaces to be scanned ill 10 ．． -21
w Light flux incident on rotating rotating mirror y direction Light flux radius = 0. Reference λO relation y direction Wavefront curvature radius = j73ri U Light source...H・-ca Laser condensing lens...f=3jOwa (f・θ lens) Cylindrical lens...f=≠4', 4'/ Iu cylindrical lens...
Distance between scanned surfaces Reference 1. ! Incident light flux to the fru rotating polygon mirror y direction Luminous flux radius = 0.260 m5 y direction Wavefront curvature radius = 10 The characteristics that the conventional device does not have are as follows.

Light source...H/-N/laser Incident light flux to rotating polygon mirror in one direction Light flux radius = 0. JO/way y direction Wavefront curvature radius = -311,, 7ψ angle (beam westra in front of rotating polygon mirror) Condensing lens... f = 3j (hss (f/θ lens) Cylindrical lens... f = jO In reality, when the optical system of a conventional device was used as the main scanning means, photographic paper T-J129 was attached to a table moving at a constant speed, and all characters were recorded, good results were obtained near the center of the scan. A record was obtained, but deterioration such as thick lines and image breakage occurred at the scanning edge.On the other hand, with the optical system of Example/1, good recording was made over the entire scanning width. When recording was performed on an electrophotographic system using the optical system of Example 2 as an exposure system, good recording was performed over the entire width.

Note that although the beam shaping means is not shown in Figure 1, this beam shaping means is, for example, a beam expanding method using a spherical lens that receives the laser beam from the laser l≠, as shown in Figures 2 and 10. Rig, a double focus cylindrical lens 16 and a short focus cylindrical lens into which the laser beam after passing through the beam expanding means/j is incident. In addition, as shown in Figure 1, there is a single cylindrical lens that receives the laser beam emitted from the laser F via the modulator I, and a spherical surface that the laser beam enters after passing through the cylindrical lens. It can also be constructed from a beam expanding means 20 consisting of a lens. The reason why all the light beams are incident on the rotating polygon mirror II with such a luminous flux is that the radius of curvature of the wavefront in one direction of the light beam incident on the rotating polygon mirror // is very large, and the light r-5 width is small. Therefore, rounding is used to prevent the optical path length from becoming extremely long. Furthermore, although a cylindrical lens was used as an optical element having a convergence effect only in one direction in Fig. 1, etc., as shown in Fig. 7, a concave cylindrical reflecting mirror! Of may also be used.

As explained in detail above, in the present invention, the reflecting surface of the rotating polygon mirror and the scanning surface are an optical system consisting of a condensing lens and a cylindrical lens, and are almost geometrically conjugate in the y direction. This prevents the occurrence of scanning pitch unevenness due to angle errors of the rotating polygon mirror, and at the same time forms a y-direction beam ray rest determined by wave optics between the condenser lens and the cylindrical lens. , thereby making the spot size uniform on the scanned surface. This was made possible by taking advantage of the fact that, while the traveling direction of the chief ray of a light beam is determined by geometric optics as mentioned above, the convergence and divergence of the light beam is determined by wave optics. . Further, according to the present invention, a low-nK, low-cost rotating polygon mirror can be used, and there is no need to use an expensive toroidal lens. Therefore, a scanning reading device or a recording device with excellent performance can be constructed at low cost.

[Brief explanation of drawings]

Fig. 1 is a basic configuration diagram of the main parts of the light beam scanning device, Fig. 2 is an explanatory diagram of the tilt angle error, Figs. 3 to 7 are explanatory diagrams of the conventional device, and Fig. The configuration diagrams of the apparatus, Figures 1 to 11 are explanatory diagrams of the beam shaping means, and Figure 1 is an explanatory diagram of another embodiment of the present invention. ≠~r, i3. tt, i7. iri...cylindrical, /zu λ
, 1... Laser lj, λ0... Beam expansion means IT... Variable V4 8P... Scanned surface Patent applicant Roku Konishi Photo Industry Co., Ltd. Agent Masujii 14 Fujiji

Claims (5)

[Claims] (1) In a light beam scanning device that performs scanning by reflecting a light beam from a light source on a rotating polygon mirror and applying it to a scanned surface via a condensing lens, between the light source and the rotating polygon mirror,
A beam shaper IR that forms a wide, flat, and substantially parallel light beam in the scanning direction is provided, and an optical element that has a light convergence effect only in a direction perpendicular to the scanning direction is provided near the scanned surface. Further, the reflecting surface of the rotating polygon mirror and the surface to be scanned are configured to have a geometrically optical conjugate relationship, and furthermore, with respect to the direction of force perpendicular to the scanning direction, the condenser lens and the optical element An optical beam scanning device configured such that a first beam waist is generated between the two and a second beam waist is generated near the surface to be scanned. (2) The light beam scanning device according to claim 1, wherein the light beam is a laser beam. (3) The light according to claim 1 or 2, characterized in that the optical element is a cylindrical lens or a concave reflecting mirror whose light convergence effect is uniform over the scanning direction. Beam scanning device 0 (4) The beam shaping means includes a beam expanding means using a spherical lens, and a long focal length cylindrical lens and a short focal length cylindrical lens into which the light beam after passing through the beam expanding means is incident. A light beam scanning device according to claim 1, 's2 or 5g3. (5) The beam shaping means includes a single cylindrical lens and an enlarging means made of a spherical lens into which the light beam after passing through the cylindrical lens enters. The light beam scanning device according to item 1, item 2, or item 3.